Silicone rubbers in the cured state have good electrically and thermally insulating properties, stable electric properties and flexibility. They are useful as potting and encapsulating materials for electric and electronic parts, and coating materials for protecting control circuit devices such as power transistors, ICs and capacitors from external, thermal and mechanical damages. In particular, gel-like cured products are best suited in these applications because of their low modulus.
Typical examples of silicone rubber compositions forming such cured gel products are organopolysiloxane compositions of the addition curing type. The organopolysiloxane compositions of the addition curing type are known, for example, from JP-A 56-143241, 63-35655 and 63-33475 as comprising an organopolysiloxane having a vinyl group attached to a silicon atom and an organohydrogenpolysiloxane having a hydrogen atom attached to a silicon atom, wherein crosslinking reaction takes place in the presence of a platinum group catalyst to form a silicone gel.
A fluorosilicone gel composition comprising an organopolysiloxane having trifluoropropyl groups is known from JP-A 7-324165.
However, silicon gels resulting from such organopolysiloxane compositions of the addition curing type are prone to degradation or swelling upon exposure to such chemicals as strong bases and strong acids or such solvents as toluene, alcohols and gasoline, often failing to maintain their performance.
As one solution to this problem, JP-A 11-116685 discloses a fluorochemical gel composition comprising a polyfluoro compound having two alkenyl groups and a divalent perfluoropolyether group in a molecule, an organohydrogenpolysiloxane having a hydrogen atom attached to a silicon atom, and a platinum group catalyst as well as a fluorochemical gel product resulting from curing thereof.
The above-described gel, however, when used as a protective agent, fails to achieve satisfactory coating/protecting effects upon contact with strong acids or strong bases in gas or liquid state. Due to the unsatisfactory coating/protecting effects, electric and electronic parts can be attacked by acids or bases, failing to offer their own performance or becoming inoperative. It is also pointed out that under stresses resulting from varying pressure and due to thermal cycling, free oil ingredients bleed out of the gel, contaminating adjacent electric and electronic parts. Such contamination also gives rise to a problem that electric and electronic parts fail to offer their own performance or become inoperative. When it is desired to apply another material to the cured gel, the influence of adversely affecting adhesion and wetting is a problem. Further, when the cured products are used with electric and electronic parts to be operated at low temperatures (around or below −30° C.), the elastic behavior at low temperatures is inferior because the composition has a glass transition temperature (Tg) of approximately −45° C. (embrittlement temperature approximately −20° C.). As a result, a problem occurs in cold areas that electric and electronic parts fail to offer their own performance or become inoperative. Especially for electric and electronic parts to be mounted on automobiles, the operation at low temperatures (below −30° C.) is of significance, suggesting a need for a material having a Tg below −70° C. (embrittlement temperature below −30° C.). In the prior art, silicone (Tg −120° C.) and fluorosilicone (Tg −75° C.) are used as the coating material for electric and electronic parts to be used in a freezing environment. These silicones both have a Tg below −70° C. and exhibit good elastic behavior at low temperatures.
It would be desirable to have a curable composition which cures into a product having chemical resistance, minimized bleeding and improved low-temperature characteristics including a Tg of −70° C. or below.